Radiation Dose Associated with Common Computed Tomography Examinations and the Associated Lifetime Attributable Risk of Cancer

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WEB-ONLY CONTENT Radiation Dose Associated With Common Computed Tomography Examinations and the Associated Lifetime Attributable Risk of Cancer

Rebecca Smith-Bindman, MD; Jafi Lipson, MD; Ralph Marcus, BA; Kwang-Pyo Kim, PhD; Mahadevappa Mahesh, MS, PhD; Robert Gould, ScD; Amy Berrington de Gonza´lez, DPhil; Diana L. Miglioretti, PhD

Arch Intern Med. 2009;169(22):2078-2086 DOSIMETRY QUANTITIES defined as joules per kilogram. The unit of gray can be used AND GLOSSARY OF TERMS for any type of radiation, but it does not describe the biological effects of different types of radiation. When radiation interacts with biological matter, the re- Radiation Weighting Factor: A dimensionless factor sulting biological effect depends on the amount of ra- by which the organ absorbed dose (rad or gray) is mul- diation energy absorbed into the material and the type tiplied to obtain a quantity that expresses, on a common of radiation. Organ dose or organ-specific absorbed dose scale for all ionizing radiation, the biological damage (rem is defined as the energy imparted per unit mass of an or- or sievert) to an exposed person. It is used because some gan or tissue. The “absorbed dose” is measured in grays types of radiation, such as alpha particles, are more bio- (Gy), defined as joules per kilogram. The gray replaced logically damaging internally than others. It is used to the rad (radiation absorbed dose), the traditional unit of derive the equivalent dose from the absorbed dose aver- absorbed dose, which is equal to 0.01 Gy. aged over a tissue or organ. Different types of radiation (eg, alpha, beta, neutron, Equivalent Dose: A quantity used in radiation pro- photon) result in different amounts of biological dam- tection to place all radiation on a common scale for cal- age, even when the absorbed doses are the same. The greater culating tissue damage. This relates the absorbed dose the rate at which the radiation transfers energy to tissue, in human tissue to the effective biological damage of the the greater the biological damage. The term equivalent dose radiation. Equivalent dose is the absorbed dose in grays was introduced to reflect the different biological effects of times the radiation-weighting factor. The radiation- different radiation types. The equivalent dose is calcu- weighting factor accounts for differences in radiation ef- lated by multiplying “absorbed dose” by “radiation weight- fects caused by different types of ionizing radiation. Some ing factor.” The radiation-weighting factor is unity for the radiation, including alpha particles, causes a greater type of radiation (photons) that comes from conven- amount of damage per unit of absorbed dose than other tional radiography and computed tomography (CT), and radiation. The sievert is the unit used to measure equiva- therefore, equivalent dose and absorbed dose are the same lent dose. for radiation exposure from CT scans. The equivalent dose Tissue Weighting Factor: The factor by which the is measured in sieverts (Sv). The traditional unit of equiva- equivalent dose in a tissue or organ is weighted to rep- lent dose was the roentgen equivalents human (or mam- resent the relative contribution of that tissue or organ to mal) (rem) (1 rem=0.01 Sv). the total health detriment resulting from uniform irra- When only a part of the body is exposed (a common diation of the body. situation from medical radiation exposure), the biologi- Effective Dose: A dosimetry quantity useful for com- cal damage depends on the exposed organ’s sensitivity paring the overall health effects of nonuniform expo- to that radiation. The International Commission on Ra- sure in terms of an equivalent to whole body exposure. diation Protection (ICRP) introduced the concept of a It takes into account the absorbed doses received by vari- tissue-weighting factor, which represents the relative con- ous organs and tissues and weighs them according to tribution of each tissue or organ to the total effects re- present knowledge of the sensitivity of each organ to ra- sulting from uniform irradiation of the whole body. There- diation. It also accounts for the type of radiation and the fore, effective dose is defined as the tissue-weighted sum potential for each type to inflict biological damage. The of the equivalent doses to specified organs and tissues unit of effective dose is the sievert. of the body. The effective dose reflects the radiation effects of a nonuniform exposure in terms of an equiva- ESTIMATING EFFECTIVE RADIATION DOSE lent whole body exposure. Therefore, effective dose can be used to compare radiation exposure across the differ- Because dose modulation techniques (automatic pro- ent types of CT studies and across the different medical grams that adjust settings throughout the CT examina- study modalities. tion to minimize radiation dose) were used on many of Absorbed Dose: The amount of energy absorbed by ion- the patient scans in our study, technical parameters needed izing radiation in a unit mass of tissue. The unit is the gray, for more detailed dose estimation could not always be

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2.5

2.0

1.5

1.0 Lifetime Attributable Risk per 1000 Patients

0.5

0 Head Chest Abdomen and Pelvis

eFigure. Lifetime attributable risk (LAR) of cancer associated with a sample of computed tomographic examinations, estimated using scanner-derived dose-length product (DLP), ImPACT CT dosimetry (computer simulation [ImPACT]), and scanner-derived DLP with adjustment (DLP adjusted).

easily extracted. Our method that relies on dose-length at the time of exposure (ages 20, 40, and 60 years). For product automatically accounts for the variation in dose each examination, we used the organ-specific radiation due to modulation software. However, for a random sub- doses generated using the ImPACT CT software, com- set of 18 patients who had undergone a head, chest, or bined with the age and organ-specific Biological Effects abdominal CT examination, we compared the dose es- of Ionizing Radiation (BEIR) VII risk estimates,7 to gen- timates from our approach to a more detailed approach erate a LAR for each type of cancer. We then summed based on a commercially available computer software pro- the LARs for each individual cancer to calculate the LARs gram (ImPACT CT Patient Dosimetry Calculator ver- for all cancers. We found moderate levels of concor- sion 0.99x).17 This involved abstracting dose data within dance between these 2 methods of estimating the LAR each patient’s scan for each set of images that used a simi- of cancer: head (rc =0.77; 95% CI, 0.65-0.85), chest lar mAs (milliamperes per second) setting and was thus (rc=0.65; 95% CI, 0.43-0.80), and abdomen (rc=0.79; 95% very time consuming, given the widespread use of dose CI, 0.62-0.89). In the head, our method of estimating LAR modulation techniques. Similar to a prior report,37 we based on using total effective dose slightly overesti- found high levels of agreement in the 2 methods for es- mated risk in both men and women; in the chest, our timating effective dose (concordance correlation coeffi- method underestimated risk, especially in women; and cient [rc]=0.94; 95% confidence interval [CI], 0.84- in the abdomen, our method overestimated risk in women 0.98). only (eFigure). To improve agreement, we developed an adjustment to our method of estimating LAR. To ad- ESTIMATING LIFETIME just the LARs, we multiplied the sex-specific mean of the ATTRIBUTABLE RISK OF CANCER ratio of the organ-specific method to our method (head, 0.66 for women and men; chest, 1.9 for women and 1.2 We compared 2 methods of estimating the lifetime at- for men; and abdomen, 0.79 for women and no adjust- tributable risks (LARs) of cancer, one based on using total ment for men). The adjusted LAR of cancer showed an even effective dose and the second using organ-specific dose. higher degree of agreement with the organ-specific ap- These 2 methods of assessing the LAR of cancer were com- proach in the head (rc=0.95; 95% CI, 0.91-0.98), chest pared in a random subset of 18 patients who had under- (rc=0.98; 95% CI, 0.96-0.99), and abdomen (rc=0.91; gone a head, chest, or abdominal CT examination, and 95% CI, 0.84-0.95) and was used to calculate the LARs of we estimated the LAR of cancer based on sex and 3 ages cancer.

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